Core Web Vitals are Google’s three field metrics for real-user experience: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). Search and Page Experience programmes use the 75th percentile of Chrome UX Report (CrUX) data when it exists. Watcher also shows lab LCP, INP, and CLS from each PageSpeed Insights run so you can debug deploys before field percentiles move.
This page is a short reference. Deep causes and fixes live on the blog tag archives (linked below). Product vocabulary (organisation, budget, Attention-Free Score) is in the Watcher Glossary.
Largest Contentful Paint (LCP)
LCP measures when the largest content element in the viewport becomes visible—typically a hero image, video, or large text block. It reflects perceived load speed.
| Assessment | Threshold (current) | Note |
|---|---|---|
| Good | ≤ 2.5 s | Google’s published “good” band |
| Needs improvement | ≤ 4.0 s | Between good and poor |
| Poor |
|
Fails the good band |
Common lab causes: slow server response (TTFB), render-blocking resources, unoptimised images, late fonts.
Further reading: Blog / LCP
Interaction to Next Paint (INP)
INP measures how quickly the page responds to user input across the visit (clicks, taps, key presses)—not only the first interaction. It replaced First Input Delay (FID) as a Core Web Vital.
| Assessment | Threshold (current) | Note |
|---|---|---|
| Good | ≤ 200 ms | Google’s published “good” band |
| Needs improvement | ≤ 500 ms | Between good and poor |
| Poor |
|
Fails the good band |
Long JavaScript tasks and heavy third-party scripts are frequent culprits. In some lab views, Total Blocking Time (TBT) helps explain main-thread contention when INP is missing or hard to reproduce.
Further reading: Blog / INP
Cumulative Layout Shift (CLS)
CLS measures visual stability: how much unexpected layout movement users see. Images without dimensions, late fonts, ads, and embeds injected above existing content often drive high CLS.
| Assessment | Threshold (current) | Note |
|---|---|---|
| Good | ≤ 0.1 | Google’s published “good” band |
| Needs improvement | ≤ 0.25 | Between good and poor |
| Poor |
|
Fails the good band |
Google has announced a tighter good CLS target of 0.08 from 2026 in Search guidance. Watcher budgets and Attention-Free gates may still use the established 0.1 lab gate until product thresholds are updated—confirm the value shown in your budget form or domain report.
Further reading: Blog / CLS
Lab vs field in Watcher
| Source | What it is | Use it for |
|---|---|---|
| Lab | One Lighthouse-based run (mobile or desktop strategy) | Release checks, debugging, budgets after deploys |
| Field (CrUX) | Real Chrome users over Google’s rolling window | “Are users still failing?” and Search-oriented status |
Missing CrUX is normal for new or low-traffic URLs. See Lab data vs field data and No CrUX data for a URL.
Supporting lab metrics (not Core Web Vitals)
These appear in Lighthouse / PageSpeed Insights and help diagnosis. They are not the three Core Web Vitals ranking metrics:
| Metric | Meaning |
|---|---|
| FCP (First Contentful Paint) | When the first text or image is painted |
| TBT (Total Blocking Time) | Lab proxy for main-thread blocking; useful when tuning INP |
| TTFB (Time to First Byte) | How long until the first byte of the response |
| Speed Index | How quickly content is visually populated during load |
How Watcher uses these numbers
- Test results show lab Core Web Vitals (and field values when CrUX is present) per strategy — Run a test and read results.
- Performance budgets can set LCP, INP, and CLS thresholds — Set performance budgets.
- Domain report Attention-Free Score includes Core Web Vitals gates in its pass criteria — Attention-Free Score.